1. Technical Field
The present invention relates generally to fuel cell systems and, more particularly, to an apparatus and method for controlling a heating value of a fuel cell including a plurality of fuel cell stacks.
2. Description of the Related Art
Producing electricity and heat from fuel, fuel cells are configured in such a way that a plurality of fuel cell stacks is connected to each other to increase the capacity. Mechanically connected to each other, and receiving fuel, air and additives for thermochemical reactions, fuel cell stacks or stack modules, each of which includes a plurality of fuel cell stacks, are stacked on top of one another in a unit of several tens to hundreds so as to obtain a desired amount of electric power output.
However, if stacks or stack modules deteriorate at different rates, a deviation in the performance between the stacks or stack modules is caused as operating time passes. Furthermore, when the efficiency of some stacks is lowered due to abnormal acceleration of deterioration, a deviation in the performance between the stacks or stack modules is also induced. Alternatively, the stacks or stack modules may not be uniform in performance from production, so that a performance deviation may be present from beginning of operation.
Moreover, when a performance deviation occurs, heating values of the stacks or stack modules differ from each other, and a deviation in temperature of the stacks or stack modules is also caused. Typically, the above-mentioned construction of the stacks or stack modules is characterized in that the temperature of a stack that has a comparatively low heating value is further reduced compared to a stack that has a high heating value, as will be described below. This results in an additional change in performance of the stacks or stack modules. When such a phenomenon worsens and reaches a specific point, some stacks or stack modules are out of operation limiting conditions such as operable stack temperature or voltage. Ultimately, a shut down phenomenon is caused.
Such a phenomenon will be described with reference to FIGS. 1A through 2B.
FIG. 1A is a view illustrating deterioration spreading influence between fuel cell stacks that are connected in parallel to each other. FIG. 1B is a graph showing the voltage and current of the fuel cell stacks to illustrate the deterioration spreading influence between the fuel cell stacks that are connected in parallel to each other. This example shows an operation mode which produces constant power regardless of deterioration of a stack.
As shown in FIGS. 1A and 1B, in a fuel cell system (normal performance of [1]) in which fuel cell stacks are electrically connected in parallel to each other, if one stack temporarily deteriorates due to an impact or environmental changes (state 2 of [2]—one stack deteriorated), the deteriorated stack is changed from state 2 to state 3 after a predetermined time has passed. The normal stack, the operating point of which has been changed to state 3′, has a higher heating value than that of the deteriorated stack.
The reason why the heating value of the normal stack is higher than that of the deteriorated stack is due to the fact that while a voltage loss (ΔV) which is a voltage difference between an open circuit voltage (OCV) or Nernst potential of the normal stack or the deteriorated stack that is measured at an initial stage and a present voltage of the stack is the same, the operating current of the normal stack becomes higher than that of the deteriorated stack. As shown in FIG. 1B, the OCV is 79 V.
In detail, referring to FIG. 1B, it can be understood that when the normal stack and the deteriorated stack are respectively present in states 3′ and 3 (primary deterioration spreading at [3]), the voltage loss (ΔV) is the same between the normal stack and the deteriorated stack, but because the operating current of the normal stack is higher than that of the deteriorated stack, the heating value of the normal stack is greater than that of the deteriorated stack (heating value=voltage loss×present operating current).
That is, when the normal stack and the deteriorated state are in current states 3′ and 3, the heating value of the normal stack is a voltage loss (79−63=16 V)×24 A, and the heating value of the deteriorated stack is 16 V×17 A. As such, the heating value of the normal stack becomes greater than that of the deteriorated stack. In this case, because of a lower heating value of the deteriorated stack than that of the normal stack, the temperature (730° C. of state 3) of the deteriorated stack is reduced compared to that of state 2 (750° C.).
Thereafter, a fuel cell system including the stacks or stack modules requires a larger amount of cooling fluid to reduce the temperature of the normal stack that has been heated. The cooling fluid further reduces the temperature of the deteriorated stack which is mechanically connected to the normal stack. As the temperature of the deteriorated stack is reduced, the resistance thereof increases. Thereby, the voltage of the deteriorated stack is further reduced, thus causing a phenomenon of current being concentrated on the normal stack. The current concentration phenomenon continues until almost the same power as the initial power is obtained. Ultimately, a difference in operating current between the normal stack and the deteriorated stack is further increased (in FIGS. 1A and 1B, states 3′ and 3 are changed to states 4′ and 4, secondary deterioration spreading at [4]). Here, as shown in FIG. 1A, the initial power is 65 V×40 A (20 A+20 A), and a present power becomes 60 V×43 A (15 A+28 A).
As time passes, if such a phenomenon continues, the temperature of the deteriorated stack is further reduced, or the operating voltage thereof is reduced below the minimum reference value. As a result, the stacks, the stack modules and the fuel cell system including them may be shut down.
FIG. 2A is a view illustrating deterioration spreading influence between fuel cell stacks that are connected in series to each other. FIG. 2B is a graph showing the voltage and current of the fuel cell stacks to illustrate the deterioration spreading influence between the fuel cell stacks that are connected in series to each other. This example shows an operation mode which produces constant power regardless of deterioration of a stack.
As shown in FIGS. 2A and 2B, in a fuel cell system (normal performance of [1]) in which fuel cell stacks are electrically connected in series to each other, if one stack temporarily deteriorates due to an impact or environmental changes (state 2 of [2]—one stack deteriorated), the current of the deteriorated stack is the same as that of the normal stack, but the voltage of the deteriorated stack becomes different from that of the normal stack depending on resistances of the stacks, because the stacks are connected in series to each other.
Referring to a state of [2] of FIG. 2A and FIG. 2B, the operating current of the deteriorated stack is the same as that of the normal stack, but a voltage loss (ΔV) of the deteriorated stack is greater than that of the normal stack. Therefore, it can be appreciated that the heating value of the deteriorated stack is greater than that of the normal stack. Because of the increased heating value of the deteriorated stack, the temperature of the deteriorated stack is increased. To reduce the temperature of the deteriorated stack that has been heated, a larger amount of cooling fluid is required. The cooling fluid further reduces the temperature of the normal stack which is mechanically connected to the deteriorated stack, whereby the resistance of the normal stack is increased. Thus, the performance curve of the normal stack is decreased (primary deterioration spreading at [3]). Therefore, to obtain the same power, the operating current is further increased, resulting in an increase in the heating value of the deteriorated stack. As the heating value of the deteriorated stack is increased, additional cooling fluid is supplied. As a result, the temperature of the normal stack is further reduced so that the performance curve of the normal stack is further decreased (the operating point is changed from state 3′ to state 4′).
As time passes, if such a phenomenon continues, the temperature of the normal stack is further reduced, or the operating voltage thereof is reduced below the minimum reference value. Ultimately, the stacks, the stack modules and the fuel cell apparatus including them may be completely shut down.
To solve the above problems, an apparatus and method for controlling a heating value of a fuel cell is required, which can control the heating value of an abnormal stack or stack module before a shut down phenomenon occurs, and which allows a user to arbitrarily control the heating value of the stack or stack module to achieve a particular purpose.